Silicon wafer and method for manufacturing the same

ABSTRACT

A method for manufacturing a silicon wafer includes a step of annealing a silicon wafer which is sliced from a silicon single crystal ingot, thereby forming a DZ layer in a first surface and in a second surface of the silicon wafer and a step of removing either a portion of the DZ layer in the first surface or a portion of the DZ layer in the second surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a siliconwafer obtained by slicing a silicon single-crystal ingot produced by theCzochralski method, and to a technology appropriate for suppressing slipdislocations to improve wafer mechanical strength.

The present invention contains subject matter related to Japanese PatentApplication No. 2004-214983, filed on Jul. 22, 2004, the contents ofwhich are incorporated herein by reference.

2. Description of the Related Art

Single-crystal silicon wafers used as substrates for semiconductordevices and the like are manufactured by slicing a siliconsingle-crystal ingot and performing heat treatment, mirror polishing,and other processing. Methods for manufacturing such siliconsingle-crystal ingots include for example the Czochralski method (CZmethod). Because the CZ method can be used to manufacture single-crystalingots having large diameters and because defect control iscomparatively easy, this method is the main method employed in themethod for manufacturing the silicon single-crystal ingots.

In order to form semiconductor devices on single-crystal silicon wafers,it is required that there is no crystal defects in a device formationarea. In the case in which the crystal defects exist in the surface inwhich circuitry is formed, the defect portion may be the cause ofcircuit breakdown or other problems. A silicon single crystal pulledusing the CZ method includes a supersaturated concentration of oxygenatoms at interstitial lattice sites. This supersaturated oxygen causesan occurrence of micro defects which are called BMDs (bulk microdefects) during annealing treatment in subsequent bulk processes.

On the other hand, because such BMDs act as getters for metal impuritiesand other causes of crystal defects, a DZ-IG method is known in which anIG (intrinsic gettering) layer acting as an impurity getter is formed byannealing a silicon wafer to cause BMDs within the wafer, and a DZ(denuded zone) layer with no lower limit to the crystal defect is formedat the silicon wafer surface.

However, in subsequent manufacturing or device fabrication entailingprocesses, there is a problem that when annealing a silicon wafer inwhich such a DZ (denuded zone) layer is formed, dislocation defects(slip) tend to occur within the wafer. Particularly when annealing isperformed in a state in which the wafer is supported by a heat treatmentboat or similar, slip dislocations tend to propagate from the portionsbeing supported on the rear surface of the wafer, resulting in a damageto the wafer during manufacturing processes, and possibly culminating ina destruction of the wafer. Consequently, as disclosed in PatentReference 1, a method is proposed in which damage (contact damage) tothe rear surface of the wafer arising from contact of the silicon waferwith the heat treatment boat during a heat treatment to form a DZ layeris removed by etching or by mechanical polishing, to suppress theoccurrence of dislocation defects (slip).

However, because the silicon wafer in which the DZ layer is formed is ina state in which a oxygen concentration in the DZ zone is extremely lowdue to outward diffusion of oxygen, the wafer strength is inherentlylow, and in the case in which minute abrasions, dislocations, or thelike occur during annealing processes in the course of devicefabrication or the like, slip dislocations easily propagate. Therefore,there is a problem that the occurrence of dislocation defects (slip)cannot be adequately suppressed merely by removal of the damage layer onthe rear surface of the wafer.

Patent Reference 1: Japanese published unexamined patent application No.2002-134521

SUMMARY OF THE INVENTION

In light of the above circumstances, an object of the present inventionis to provide a silicon wafer with excellent strength characteristicsand provided with a DZ layer having extremely few crystal defects, and amethod for manufacturing the same.

In order to attain the above object a method for manufacturing a siliconwafer of a first aspect of the present invention includes a step ofannealing a silicon wafer which is sliced from a silicon single crystalingot, thereby forming a DZ layer in a first surface and in a secondsurface of the silicon wafer, and a step of removing either a portion ofthe DZ layer in the first surface or a portion of the DZ layer in thesecond surface.

The method further may include a step of removing a portion of the DZlayer formed in a circumference of the silicon wafer. It is preferablethat in the step of annealing the silicon wafer, the annealing isperformed in a non-oxidizing atmosphere at a temperature of 1000° C. orhigher. It is preferable that in the step of removing the portion of theDZ layer, in a surface portion from which the DZ layer is removed in thefirst surface or in the second surface, a concentration of oxygen ismade to be 10×10¹⁷ atoms/cm³ or higher.

The step of removing the portion of the DZ layer may be a step ofreducing a thickness of the DZ layer by mirror polishing. A siliconwafer which is manufactured by the above-described method formanufacturing a silicon wafer, is then obtained.

A method for manufacturing a silicon wafer of a second aspect of thepresent invention includes a step of forming an oxide film either in afirst surface or in a second surface of a silicon wafer which is slicedfrom a silicon single-crystal ingot, and a step of annealing the siliconwafer in which the oxide film is formed.

It is preferable that in the step of annealing the silicon wafer, theannealing is performed in a non-oxidizing atmosphere at a temperature of1150° C. or higher. A silicon wafer which is manufactured by theabove-described method for manufacturing a silicon wafer, is thenobtained.

A method for manufacturing a silicon wafer of a third aspect of thepresent invention includes a step of annealing a silicon wafer which issliced from a silicon single-crystal ingot, at a temperature of 1150° C.or higher while changing an annealing atmosphere from a non-oxidizingatmosphere to an oxidizing atmosphere, thereby forming a DZ layer and anoxide film in a first surface and in a second surface of the siliconwafer, and a step of removing either or both of a portion of the oxidefilm formed in the first surface and a portion of the oxide film formedin the second surface

It is preferable that the oxide film is formed to a thickness of 0.20 nmor more. A silicon wafer which is manufactured by the above-describedmethod for manufacturing a silicon wafer, is then obtained.

Inventors discovered the followings. When a wafer is annealed, slipdislocations may occur due to thermal stress from portions in a rearsurface of the wafer at which the wafer is supported. However, extent ofpropagation of these slip dislocations depends on a thickness of a DZlayer formed in the wafer, that is, it depends on an Oi concentration(oxygen concentration) in the rear surface (support surface duringannealing) of the wafer. The reason is thought to be as follows. Whenthe slip dislocations are formed, oxygen precipitates formed in avicinity of the dislocations fix the slip dislocations, therebydislocation propagation can be halted. However, in the case in which adensity and a size of the precipitates are insufficient for halting theslip dislocations, the slip dislocation propagation is not suppressed.This is thought to be the reason why the extent of propagation of theslip dislocations depends on the Oi concentration. Therefore, in orderto suppress the propagation of the slip dislocations which may lead toreduce a strength of the wafer, it is necessary to provide a sufficientOi concentration in a vicinity of points in which the slip dislocationsoccur in the rear surface (support surface) of the wafer.

Hence, in order to maintain the strength of the wafer, the inventorsmake the oxygen concentration in the rear surface (support surface) ofthe wafer into a prescribed value, that is, adopt either a means ofremoving a DZ layer in the rear surface of the wafer, thereby exposingthe bulk layer (BMD layer), or a means of setting the oxygenconcentration in the rear surface (support surface) of the wafer so asto be an oxygen concentration at which precipitates having necessarydensity and size to fix the slip dislocations can be formed. In order toset the oxygen concentration as described above, the inventors adopt ameans to anneal the wafer in which the rear surface has an oxide film.

Here, an ability to suppress the slip dislocations by depositing theoxide film is explained.

FIG. 7 is a typical graph showing the Oi concentration in a waferthickness direction.

As indicated by a symbol S in FIG. 7, the Oi concentration in a slicedwafer (a wafer after mirror polishing) is substantially constant. In thecase in which there is no oxide film in a wafer and the wafer issubjected to annealing for forming a DZ layer, interstitial oxygendiffuses outward (out-diffusion) from a surface of the wafer. Therefore,as shown in a symbol A in FIG. 7, the Oi concentration near the surfaceof the wafer declines. On the other hand, in the case in which an oxidefilm is formed in a wafer and the wafer is subjected to annealing, asshown in a symbol Y in FIG. 7, decline in Oi concentration near asurface of the wafer is suppressed. As a result, an oxygen concentrationis obtained at which precipitates having necessary density and size tofix the slip dislocations can be formed, and so it is thought that astate adequate to suppress the slip dislocations can be maintained.

Here, FIG. 7 does not show in detail a behavior of the Oi concentrationin a portion corresponding to a BMD layer. The oxygen concentrationsemployed in this Specification are measured according to ASTM-F121-1979.

According to the method for manufacturing a silicon wafer of the firstaspect of the present invention, by removing the portion of the DZ layerin the second surface (here, the second surface is defined as a supportsurface by which the wafer is supported during annealing), BMD layer isexposed on the second surface. Therefore, dislocations are fixed byprecipitates arising from interstitial oxygen, and dislocationpropagation is suppressed. Consequently, when annealing the wafer inorder to impart desired characteristics before forming devices on thefirst surface on which the DZ layer is exposed, even if slipdislocations occur during annealing, oxygen precipitates are formed in avicinity of the slip dislocations, thereby dislocation propagation ishalted, and a reduction in the strength of the wafer due to occurrenceof the slip dislocations is suppressed. Consequently, silicon wafers canbe obtained which have the DZ layer indispensable for devicefabrication, and which retain high mechanical strength.

According to the methods for manufacturing a silicon wafer of the secondand third aspects, by forming the oxide film in at least the secondsurface, the oxide film in the second surface is exposed, thereby adecline in the oxygen concentration in the wafer during annealing issuppressed by the oxide film. Consequently, an oxygen concentration canbe maintained which is sufficient to suppress propagation of the slipdislocations. Therefore, when the wafer is annealed in subsequentprocesses, the propagation of the slip dislocations during annealing canbe suppressed. Hence, reductions in the wafer strength due to the slipdislocations can be suppressed, and silicon wafers can be obtained whichhave the DZ layer indispensable for device fabrication, and which retainhigh mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing one of examples of a silicon waferof the present invention.

FIG. 2 is a schematic diagram showing another example of a silicon waferof the present invention.

FIG. 3A is a diagram showing a silicon single crystal pulled by a CZmethod, in an explanatory diagram illustrating an example of a methodfor manufacturing a silicon wafer of a first embodiment of theinvention.

FIG. 3B is a diagram showing a process of slicing a portion of thesilicon single crystal pulled by the CZ method into a cylindrical ingotand removing a circumferential portion, in an explanatory diagramillustrating an example of a method for manufacturing a silicon wafer ofa first embodiment of the invention.

FIG. 3C is a diagram showing a process of forming wafers from thecylindrical ingot, in an explanatory diagram illustrating an example ofa method for manufacturing a silicon wafer of a first embodiment of theinvention.

FIG. 3D is a diagram showing a process of flattening the wafer, removinga damage layer, and mirror-polishing, in an explanatory diagramillustrating an example of a method for manufacturing a silicon wafer ofa first embodiment of the invention.

FIG. 3E is a diagram showing a process of heat-treating amirror-polished wafer, in an explanatory diagram illustrating an exampleof a method for manufacturing a silicon wafer of a first embodiment ofthe invention.

FIG. 3F is a diagram showing a process of exposing a BMD layer in asecond surface of the wafer, on which devices are not formed, in anexplanatory diagram illustrating an example of a method formanufacturing a silicon wafer of a first embodiment of the invention.

FIG. 4A is a diagram showing a process of subjecting a first surface anda second surface of a wafer to a surface treatment such as a mirrorpolishing, in an explanatory diagram illustrating one example of amethod for manufacturing a silicon wafer of a second embodiment of theinvention.

FIG. 4B is a diagram showing a process of forming an oxide film in thesecond surface, in an explanatory diagram illustrating one example of amethod for manufacturing a silicon wafer of a second embodiment of theinvention.

FIG. 4C is a diagram showing a process of heat treatment, in anexplanatory diagram illustrating one example of a method formanufacturing a silicon wafer of a second embodiment of the invention.

FIG. 4D is a diagram showing a process of removing the oxide film in thesecond surface by the mirror polishing, in an explanatory diagramillustrating one example of a method for manufacturing a silicon waferof a second embodiment of the invention.

FIG. 5 is a schematic diagram showing the other example of a siliconwafer of the present invention.

FIG. 6A is a diagram showing a process of subjecting a first surface anda second surface of a wafer to a surface treatment such as a mirrorpolishing, in an explanatory diagram illustrating the other example of amethod for manufacturing a silicon wafer of a second embodiment of theinvention.

FIG. 6B is a diagram showing a process of forming an oxide film, in anexplanatory diagram illustrating the other example of a method formanufacturing a silicon wafer of a second embodiment of the invention.

FIG. 6C is a diagram showing a process of heat treatment, in anexplanatory diagram illustrating the other example of a method formanufacturing a silicon wafer of a second embodiment of the invention.

FIG. 6D is a diagram showing a process of removing a portion of theoxide film in the second surface by the mirror polishing, in anexplanatory diagram illustrating the other example of a method formanufacturing a silicon wafer of a second embodiment of the invention.

FIG. 7 is a graph showing an Oi concentration in a wafer depthdirection, to explain an action of an oxide film in a silicon wafer ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, embodiments of the invention are explained based on the drawings.

FIG. 1 is a schematic diagram showing one of examples of a silicon waferof the present invention. Here, it should be noted that to facilitateexplanation of the silicon wafer shown, a thickness has beenexaggerated. On this silicon wafer 10, a DZ (denuded zone) layer 12 isexposed on a first surface (device fabrication surface) on which devicesare subsequently to be fabricated, and similarly, the DZ (denuded zone)layer 12 is exposed on a circumferential surface 15 and on a bevelededges 16, 17. Further, a BMD (bulk micro defect) layer 14 is exposed ona second surface 13 which is a surface supported during annealing and isalso a rear surface opposite the device fabrication surface.

The DZ layer 12 is a layer in which crystal defects do not exist, and ona surface thereof, devices are fabricated in subsequent processes. Inthe BMD layer 14, an oxygen concentration near the second surface 13 isfor example 10×10¹⁷ atoms/cm³, and the BMD layer 14 acts as a getteringlayer (IG layer). This silicon wafer 10 is characterized in that the BMDlayer 14 is exposed on the second surface 13 which is the rear surfaceopposite the device fabrication surface.

In the second surface 13 which is supported by a heat treatment jig andthe like during the annealing described below, it is preferable that theoxygen concentration is 8×10¹⁷ atoms/cm³ or higher, and more preferablethat the concentration is 10×10¹⁷ atoms/cm³. In the case in which theabove oxygen concentration is lower than 8×10¹⁷ atoms/cm³, precipitatescannot be formed sufficiently, slip dislocations propagate, and thewafer strength is reduced, which is undesirable. And in the case inwhich the oxygen concentration is lower than 10×10¹⁷ atoms/cm³,precipitates to suppress slip dislocation propagation are at leastformed, however in in-plane directions of the wafer, there is apossibility that a density distribution of oxygen occurs to causeinsufficient precipitate formation in some areas, which is undesirable.

In this silicon wafer 10, because the BMD layer 14 is exposed on thesecond surface 13, even in the case in which dislocation defects (slip)occur in the second surface 13 during heat treatment in subsequentprocesses, propagation can be suppressed. This is due to the fact that amovement of the dislocations in silicon depends greatly on aninterstitial oxygen concentration. In a silicon wafer having a highoxygen concentration, a formation of minute precipitates on adislocation line fixes the dislocation, so that considerable thermalstress is necessary for dislocation motion. However, in the case inwhich a dislocation occurs in a DZ layer having a low oxygenconcentration, it is not expected to obtain an effect to fix thedislocations by the interstitial oxygen. A dislocation which begins tomove in the DZ layer is not stopped even in a BMD layer (bulk region)which has an oxygen concentration higher than that in the DZ layer, sothat the slip dislocation propagates. Therefore, the BMD layer 14 isneeded to be adequately exposed on the second surface 13.

During annealing, by exposing the BMD layer 14 in the second surface 13in which the slip dislocations easily occur, the dislocation propagationis suppressed by the precipitates formed from the interstitial oxygenpresent in the vicinity of the second surface 13. Therefore, a reductionin strength of the silicon wafer 10 is suppressed even after a heattreatment in subsequent processes. The silicon wafer 10 can be obtainedwhich includes a DZ layer 12 and maintains high mechanical strength. Andeven when the silicon wafer 10 thus manufactured is subjected to a heattreatment during a device fabrication process, reductions in strengthare similarly suppressed. Therefore, excellent strength characteristicsare obtained even while the wafer has the DZ layer 12.

Here as shown in FIG. 2, a structure may also be adopted in which a BMDlayer 24 is exposed on a circumferential surface 25 of a silicon wafer20.

This silicon wafer 20 has a DZ layer 22 exposed on a first surface 21(device fabrication surface) on which devices are to be fabricated insubsequent processes, and the DZ (denuded zone) layer 22 is also exposedon a first beveled surface 26. In addition, a BMD layer 24 is exposed ona second surface 23 which is a rear surface opposite the devicefabrication surface, and the BMD layer 24 is also exposed on thecircumferential surface 25 and a second beveled surface 27. Here, the DZlayer 22 is exposed on a first-surface side 21 of the circumferentialsurface 25 of the silicon wafer 20.

By causing the BMD layer 24 to be exposed on the second surface 23 inthe silicon wafer 20, dislocation propagation is suppressed byprecipitates. Furthermore, because the BMD layer 24 is exposed on thecircumferential surface 25 of the silicon wafer 20, dislocationpropagation in the circumferential surface 25 and the beveled surface 27of the silicon wafer 20 is also suppressed. By this means, in thesilicon wafer 20, a necessary and sufficient strength can be maintainedin the circumferential surface 25 and the beveled surface 27 which tendto receive shocks in subsequent processes. Therefore, a risk of damageis reduced and still greater strength of the silicon wafer 20 can bemaintained.

Next, a method for manufacturing a silicon wafer of the first embodimentof the invention, applied to manufacture a silicon wafer having theabove-described configuration, is explained.

FIG. 3A through FIG. 3F are explanatory diagrams illustrating the methodfor manufacturing a silicon wafer of the first embodiment of theinvention. A silicon single crystal C is pulled by the CZ method, and aportion of the silicon single crystal C is cut into a cylindrical ingotN, and a circumferential portion is removed (FIG. 3B). This cylindricalingot N is sliced at a prescribed thickness to obtain numerous wafers(silicon wafers) 30 a (FIG. 3C). A first surface 31 and a second surface33 of the wafer 30 a are subjected to lapping, beveling, etching, andother processes to flatten the wafer surfaces and remove damage layers,followed by mirror polishing (FIG. 3D). Here, in FIG. 3A through FIG.3F, wafers in a beveled state are omitted.

A mirror-polished wafer 30 a is then subjected to annealing to controloxygen precipitation. In this process, annealing is performed in anon-oxidizing atmosphere at a temperature of 1000° C. or higher. In thewafer 30 a which has been subjected to this heat treatment, a BMD layer34 is formed which makes up an intrinsic gettering (IG) region in acenter portion, as shown in FIG. 3E, and also, a DZ (denuded zone) layer32 is formed in the first surface 31, second surface 33, and acircumferential surface 35.

Next, as shown in FIG. 3F, in the wafer 30 a, the second surface 33 onwhich no devices are to be formed is subjected to mirror polishing so asto remove a portion of the DZ layer 32 in the second surface 33, and theBMD layer 34 is exposed on the second surface 33. Removal of the portionof the DZ layer 32 may be performed by polishing to reduce a thicknessof the portion of the DZ layer 32 until the oxygen concentration in thesurface of the second surface 33 is at least 8×10¹⁷ atoms/cm³ or higher,and preferably 10×10¹⁷ atoms/cm³ or higher. By means of this DZ layerremoval process, a silicon wafer 30 is formed in which the DZ layer 32is exposed on the first surface 31, and the BMD layer 34 is exposed onthe second surface 33.

In this silicon wafer 30, when annealing is performed to impart desiredcharacteristics prior to a formation of devices on the first surface 31on which the DZ layer 32 is exposed, the BMD layer 34 is exposed on thesecond surface 33. As a result, dislocation propagation is suppressed byprecipitates in the BMD layer 34, thereby a reduction in wafer strengthdue to slip dislocations is suppressed. In this way, the silicon wafer30 is obtained which includes the DZ layer 32 indispensable for deviceformation and has high mechanical strength.

While omitted from the above explanation, by further performing abeveling process, silicon wafers are manufactured with a structuresimilar to that of the silicon wafer 10 and having the beveled edges 16,17 as shown in FIG. 1.

It is preferable that, when the portion of the DZ layer 32 in the secondsurface 33 is removed by the mirror polishing as shown in FIG. 3F, aportion of the DZ layer 32 in a circumferential surface 35 of the wafer30 a is also removed to expose the BMD layer 34 on the circumferentialsurface 35 as well. Thereby dislocation propagation from thecircumferential surface 35 of the silicon wafer 30 is suppressed, andthe strength of the silicon wafer 30 can be made still higher.

While omitted from the above explanation, by further performing abeveling process, silicon wafers are manufactured with a structuresimilar to that of silicon wafers 20 and having the beveled edges 26, 27as shown in FIG. 2.

In order to remove the portion of the DZ layer 32, as long as thesurface is made in a mirror-polished state in the end, it is alsopossible to employ other means in addition to mirror polishing, or othermeans instead of mirror polishing. For example, a method in which aprocess of laser etching, surface polishing, or wet etching is performedin advance, followed by mirror polishing can be employed.

Next, a method for manufacturing a wafer of a second embodiment of theinvention is explained, based on the drawings.

FIG. 4A through FIG. 4D are explanatory diagrams illustrating oneexample of the method for manufacturing a silicon wafer of the secondembodiment of the invention.

In the method for manufacturing a silicon wafer 50 of this embodiment,before a step corresponding to the step of forming the DZ layer 32 inthe first embodiment as shown in FIG. 3D and FIG. 3E, a step of formingan oxide film 58 in a second surface 53 which is a rear surface oppositea device fabrication surface is employed. This is a difference betweenthe first embodiment and the second embodiment.

Similarly to the first embodiment as shown in FIG. 3, a siliconsingle-crystal ingot pulled using the CZ method is sliced at aprescribed thickness to obtain wafers (silicon wafers) 50 a, a firstsurface 51 and a second surface 53 of the wafer 50 a are subjected tosurface treatment such as mirror polishing (FIG. 4A).

Next, as shown in FIG. 4B, prior to performing heat treatment, the oxidefilm 58 is formed in the second surface 53. It is preferable that thisoxide film 58 is formed at a thickness of for example 20 nm or greater.The oxide film 58 may be formed by heating the wafer 50 a in anoxidizing atmosphere to form the oxide film over the entirety of thewafer 50 a, followed by selectively removing only a portion of the oxidefilm in the first surface 51 using hydrofluoric acid and the like.Alternatively, the oxide film 58 may be formed by using other means suchas selectively forming the oxide film 58 only in the second surface 53.

The wafer 50 a in which the oxide film 58 is formed only in the secondsurface 53 is subjected to annealing to occur IG (intrinsic gettering).This IG may be induced by for example annealing in a non-oxidizingatmosphere at a temperature of 1150° C. or higher. By the annealingprocess, as shown in FIG. 4D, a DZ layer 52 is formed in the firstsurface 51 and a circumferential surface 55, and also a BMD (bulk microdefect) layer 54 which acts as a gettering layer in the IG spreads inthe wafer, resulting in a wafer 50 a having an oxide film 58 formed inthe second surface 53.

Next, the oxide film 58 in the second surface 53 is mirror-polished andremoved. Here, the oxide film 58 can be selectively removed usinghydrofluoric acid or the like.

By this means, a silicon wafer with configuration similar to that of theone example shown in FIG. 1 is manufactured.

As shown in FIG. 4D, a DZ (denuded zone) layer 52 is exposed on thefirst surface 51 of this silicon wafer 50 on which devices are to befabricated in subsequent processes. And, a BMD (bulk micro defect) layer54 which acts as a gettering layer in the IG, is exposed on the secondsurface 53 which is supported by a support jig and the like during heattreatment.

According to this method for manufacturing a silicon wafer 50, becausethe oxide film 58 is formed in the second surface 53, the second surface53 can be covered by the oxide film 58 which has an extremely highoxygen concentration in heat treatment to form the DZ layer 52 or otherheat treatments. By this means, during heat treatment to form the DZlayer 52, a sufficient Oi concentration is maintained in the secondsurface 53 and precipitates having sufficient density and size tosuppress dislocation propagation can be obtained in a vicinity of slipdislocations.

As a result, in heat treatment to form a DZ layer 52 or other heattreatments among the processes to manufacture the silicon wafer 50, evenwhen dislocation defects (slip) occur in the second surface 53, oxygenprecipitates are formed in a vicinity of dislocation lines, therebypropagation is suppressed. That is, by forming the oxide film 58,dislocation propagation is suppressed. Because dislocation propagationin the silicon wafer 50 is suppressed in this way, reductions in thestrength of the silicon wafer 50 due to heat treatment in subsequentprocesses are suppressed. Thus, the silicon wafer 50 having the DZ layer52 with high mechanical strength can be obtained.

Furthermore, the BMD layer 54 is exposed on the second surface 53 of thesilicon wafer 50 which is manufactured in this way. Therefore, similarlyto the first embodiment, even when heat treatment is performed tofabricate devices and the like on the silicon wafer 50, reduction instrength of wafer is suppressed. Consequently, excellent strengthcharacteristics are obtained even while having a DZ layer 52.

FIG. 5 is a schematic diagram showing the other example of a siliconwafer of the present invention.

As shown in FIG. 5, in this silicon wafer 60 a DZ layer 62 is exposed ona first surface 61, and a BMD layer 64 is exposed on a second surface63, a circumferential surface 65, and beveled surfaces 66, 67. By thusexposing the BMD layer 64 on the circumferential surface 65 and on thebeveled surfaces 66, 67, dislocation propagation from thecircumferential surface 65 and from the beveled surfaces 66, 67 of thesilicon wafer 60 is suppressed, and the strength of the silicon wafer 60can be further improved, which is preferable.

In order to manufacture the above-described silicon wafer, the methodfor manufacturing a silicon wafer of the second embodiment of theinvention is explained. FIG. 6A through FIG. 6D are explanatory diagramsillustrating the other example of the method for manufacturing a siliconwafer of the second embodiment of the invention.

Similarly to the first embodiment as shown in FIG. 3, a siliconsingle-crystal ingot pulled using the CZ method is sliced at aprescribed thickness to obtain wafers (silicon wafers) 60 a. A firstsurface 61, a second surface 64, a circumferential surface 65, andsurfaces corresponding to beveled surfaces 66, 67 as shown in FIG. 5 aresubjected to mirror polishing and other surface treatment (FIG. 6A).Here, in FIG. 6A through FIG. 6D, beveled surfaces are not shown.

As shown in FIG. 6B, an oxide film 68 is then formed in the secondsurface 63, the circumferential surface 65, and the surfacescorresponding to the beveled surfaces 66, 67 of the mirror-polishedwafer 60 a. It is preferable that this oxide film 68 is formed at athickness of for example 20 nm or more. The oxide film 68 may be formedby heating the wafer 60 a in an oxidizing atmosphere to form the oxidefilm over the entirety of the wafer 60 a, followed by selectivelyremoving only a portion of the oxide film in the first surface 61 usinghydrofluoric acid and the like. Alternatively, the oxide film 58 may beformed by using other means such as selectively forming the oxide film68 only in the second surface 63, the circumferential surface 65, andthe surfaces corresponding to the beveled surfaces 66, 67.

The wafer 60 a in which the oxide film 68 is formed only in the secondsurface 63, the circumferential surface 65, and the surfacescorresponding to the beveled surfaces 66, 67 is subjected to annealing.By the annealing process, in a surface portion in which the oxide film68 is located and a deep portion in the wafer, precipitation nucleiwhich can become IG (intrinsic gettering) sites are precipitated, and ina surface portion in which the oxide film 68 is not located,interstitial oxygen diffuses outward to form a DZ layer 62. Thisannealing should be performed for example in a non-oxidizing atmosphereat a temperature of 1150° C. or higher. In the second surface 63, thecircumferential surface 65, and the surfaces corresponding to thebeveled surfaces 66, 67, because the oxide film 68 is present, oxygendiffuses inward (in-diffusion) from the oxide film 68 which is a sort ofa oxygen supply source. Therefore, the decline in oxygen concentrationis more gradual than that in a portion in which the DZ layer 62 isformed.

As a result, as shown in FIG. 6C, by this annealing process, the wafer60 a is fabricated which has the DZ layer 62 formed in the first surface61 and a BMD (bulk micro defect) layer 64 formed in an interior, thesecond surface 63, the circumferential surface 65, and the surfacescorresponding to the beveled surfaces 66, 67.

Next, the oxide film 68 is removed. Thereby a silicon wafer 60 ismanufactured in which the DZ layer 62 is exposed only on the firstsurface 61.

The wafer is subjected to the above-described heat treatment to form theDZ layer in a state in which the oxide film 68 is formed in the secondsurface 63, the circumferential surface 65, and the beveled surfaces 66,67. Therefore, in the heat treatment to form the DZ layer 62, asufficient oxygen concentration is maintained in the second surface 63,the circumferential surface 65, and the beveled surfaces 66, 67, therebyprecipitates can be obtained with sufficient density and size tosuppress dislocation propagation. As a result, because dislocationpropagation in the second surface 63, circumferential surface 65, andbeveled surfaces 66, 67 can be suppressed by these precipitates,reduction of the wafer strength due to slip dislocations is suppressed.Thus, the silicon wafer 60 having the DZ layer 62 indispensable fordevice fabrication with high strength can be obtained.

Here, when performing annealing for forming the oxide film, the oxidefilm may be formed in the first surface and the second surface of thesilicon wafer by annealing for example at a temperature of 1150° C. orhigher in an atmosphere which is changed from non-oxidizing tooxidizing. Thereafter, only a portion of the oxide film in the firstsurface, on which devices are to be fabricated, is removed.

Depending on product specifications, it may be possible to omit removalof the oxide film formed during annealing. That is, a silicon wafer canbe fabricated having an oxide film in portions other than a DZ layer, ina second surface alone, or in portions excluding at least the DZ layerwhich is to be a device area.

According to this silicon wafer, because the oxide film is present inthe second surface, even in the case in which the silicon wafer issubjected to heat treatment to fabricate devices, a sufficient Oiconcentration can be maintained in the second surface. Hence,precipitates having adequate density and size to suppress dislocationpropagation can be obtained in a vicinity of slip dislocations. As aresult, even when dislocation defects (slip) occur in the secondsurface, oxygen precipitates are formed in a vicinity of a dislocationline, and propagation is suppressed. Thus, excellent strengthcharacteristics are obtained while having the DZ layer 62.

EXAMPLES Example 1

The inventors verified an influence of the DZ layer on a wafer strength(or slip dislocation). In order to perform the verification, thefollowing wafers were prepared: a mirror-polished wafer prior toannealing (Sample 1); a wafer annealed at 1200° C. for one hour in Ar(Sample 2); a wafer which is annealed in Ar, followed by polishing toremove a portion of 10 μm thick from a rear surface (second surface)(Sample 3); and a wafer which is annealed in Ar, followed by polishingto remove a portion of 30 μm thick from a rear surface (Sample 4).

Resistivity of all the wafers was in a range from 3 to 11Ω·cm. Thesewafers were subjected to heat treatment for thermal stress loading usinga vertical furnace under a condition in which the wafers were set intothe furnace at 900° C., temperature was ramped up at 10° C./min to 1150°C., held at 1150° C. for 30 min, and ramped down at 3° C./min to 900°C., and the wafers were removed at 900° C. A surface in contact with aheat treatment boat was a rear surface (second surface) of the wafer.The rear surface of the wafer after heat treatment was observed by anX-ray topography. From the observation, it was revealed that slipdislocations had propagated from traces of the wafer boat. States ofpropagation of slip dislocation for the different samples are shown inTable 1. Table 1 also shows bulk oxygen concentrations of the samplesprior to thermal stress load testing, and results of SIM measurementsfor the oxygen concentrations in a vicinity of the rear surface of thewafers. Here, “vicinity of the surface” means a portion ranging from thesurface to approximately 0.5 μm thick at which an effect ofcontamination is small; however, thickest limit of the portion is notexactly 0.5 μm thick, so long as the effect of contamination is small.

TABLE 1 Oxygen concentration Bulk oxygen in rear surface Extent ofSample concentration measured by SIMS slip No. DZ layer (×10¹⁷ atoms/cc)(×10¹⁷ atoms/cc) dislocation Sample none 12.6 12.4 A 1 Sample present11.2 0.7 C 2 Sample present 11.1 7.8 B 3 (10 μm polished) Sample present11.0 10.7 A 4 (30 μm polished) (A: Almost no slip occurrence, B: someslip occurrence, C: extensive slip dislocation occurrence)

From the verification results shown in Table 1, the higher the oxygenconcentration in the rear surface (second surface), the less theoccurrence of slip dislocations. It is confirmed that by exposing a BMDlayer or an oxide film which has high oxygen concentration on the secondsurface of the silicon wafer, slip dislocations are reduced and thestrength of the silicon wafer is greatly enhanced.

Example 2

Next, the inventors verified a suppression of slip dislocations due to aformation of an oxide film. Outward diffusion of oxygen in an annealingprocess can also be prevented by forming the oxide film to protect therear surface of the wafer prior to annealing. This is because the oxygenconcentration at a surface interface between the oxide film and thesilicon is constantly maintained at a thermal equilibrium concentration.At a temperature of 1150° C. or higher, the thermal equilibriumconcentration of interstitial oxygen is approximately 8×10¹⁷ atoms/cc,and an effect to prevent propagation of slip dislocation can beexpected. It is desirable that the oxide film formed before annealinghas a thickness of 20 nm or thicker. The reason is as follows. In thecase in which the thickness of the oxide film is thinner, the oxide filmeasily dissociates from the surface in heat treatment in a non-oxidizingatmosphere such as Ar or H₂, however in the case in which the thicknessis 20 nm or more, the oxide film can be sufficiently stable against theannealing process.

A formation of the oxide film is effectively conducted during theannealing process. By changing the gas atmosphere from a non-oxidizingatmosphere to an atmosphere containing some oxygen gas in the annealingprocess, the wafer surface is oxidized. Interstitial oxygen isin-diffused from a thermal oxide film formed at this time, thereby aconcentration of interstitial oxygen in the wafer surface can be raised.

In order to investigate an effect of the oxide film on the strength (orslip dislocations) of an annealed wafer, wafers were prepared asfollows. An oxide film having a thickness of approximately 20 nm wasgrown at 1000° C. on mirror-polished wafers prior to annealing, and aportion of the oxide film only in a front surface was then removed usinga diluted HF solution. The wafers were annealed in Ar at 1200° C. forone hour (Sample 5) or at 1150° C. for two hours (Sample 6). Also,wafers (Sample 7 and Sample 8) were prepared by annealing in Ar at 1200°C. for one hour, followed by changing temperature in a furnace to 1150°C. and holding the temperature to anneal in 95% Ar+5% O₂ for three hoursand for five hours respectively. A thickness of the oxide film formed inthe rear surface by the annealing was measured, and the oxide film wasthen removed using a diluted HF solution to perform the following tests.

Resistivity of all the wafers was in a range from 3 to 11Ω·cm. Thesewafers were subjected to heat treatment for thermal stress loading usinga vertical furnace under a condition in which the wafers were, set intothe furnace at 900° C., temperature was ramped up at 10° C./min to 1150°C., held at 1150° C. for 30 min, and ramped down at 3° C./min to 900°C., and the wafers were removed at 900° C. A surface in contact with theheat treatment boat was a rear surface (second surface) of the wafer.The rear surface of the wafer after heat treatment was observed by anX-ray topography. From the observation, it was revealed that slipdislocations had propagated from traces of the wafer boat. States ofpropagation of slip dislocation for the different samples are shown inTable 2. Table 2 also shows bulk oxygen concentrations of the samplesprior to thermal stress load testing, and results of SIM measurementsfor the oxygen concentrations in a vicinity of the rear surface of thewafers.

TABLE 2 Oxygen concentration Oxide in rear surface Oxide film measuredby Extent of Sample film Anneal thickness SIMS (×10¹⁷ slip No. growthprocess (nm) atoms/cc) dislocation Sample 5 growth 1200° C./ 21.0 10.3 Abefore 1 hr anneal process Sample 6 growth 1150° C./ 23.5 8.7 B before 2hrs anneal process Sample 7 growth 1200° C./ 22.4 8.1 B during 1 hr +anneal 1150° C./ process 3 hrs Sample 8 growth 1200° C./ 35.8 8.4 Bduring 1 hr + anneal 1150° C./ process 5 hrs (A: Almost no slipoccurrence, B: some slip occurrence, C: extensive slip dislocationoccurrence)

According to the verification results shown in Table 2, occurrence ofslip dislocations was greatly suppressed in all samples 5 to 8 comparedwith Sample 1. High-temperature annealing is generally performed for thepurpose of reducing the COP and micro oxygen precipitates in a vicinity,of a surface. When micro defects in the vicinity of the surface wasobserved using a MO601 (Mitsui Mining and Smelting Co., Ltd.), numbersof defects in Samples 5 through 8 were equal to that in Sample 1, andexcellent crystallinity was confirmed.

In the above, preferred embodiments of the present invention have beenexplained, however the present invention is not limited to theseembodiments. Various additions, omissions, substitutions, and othermodifications to the configuration can be made within the scope of thepresent invention. This invention is not limited to the foregoingexplanations, but is limited solely by the scope of the attached claims.

1-9. (canceled)
 10. A method for manufacturing a silicon wafercomprising: a step of annealing a silicon wafer which is sliced from asilicon single-crystal ingot at a temperature of 1150° C. or higherwhile changing an annealing atmosphere from a non-oxidizing atmospherefor one hour or more to an oxidizing atmosphere for three to five hours,thereby forming a DZ layer and forming an oxide film in a first surfaceshich is a device fabrication surface of the silicon wafer and in asecond surface which is a rear surface opposite a device fabricationsurface of the silicon wafer; and removing the oxide film formed in thefirst surface which is the device fabrication surface.
 11. A method formanufacturing a silicon wafer according to claim 10, wherein the oxidefilm is formed to a thickness of 20 nm or more.
 12. (canceled)